Flattening the outer panel surface of a color Braun tube (color cathode ray tube) makes it possible to improve the viewability of on-screen picture images. However, in a shadow mask type of Braun tube, it is necessary for the effective surface section of the shadow mask to have a curvature in order to maintain the mechanical strength of the shadow mask. It is also important to maintain a uniform distance between the shadow mask and the phosphor screen on the inner surface of the panel. To this end, a scheme is known for giving a curvature to the inner surface of the panel opposing the shadow mask, while at the same time flattening the outer surface of the panel. However, if the glass thickness at the periphery of the panel is larger than that at the center of the panel, the following problems would occur: (1) the brightness of the screen is degraded in the periphery of the screen; and (2) the panel is difficult to manufacture. Accordingly, in cases where the outer surface of the panel is flat, it is impossible to extremely increase the curvature of the inner surface of the panel. In this case, it is impossible to give a sufficient curvature to the shadow mask, so that the strength of the shadow mask becomes a problem.
As a method of improving the strength of a shadow mask having, a small curvature (large radius of curvature), a technique for forming beads or irregular bent/curved portions within the effective surface area of the shadow mask is described in U.S. Pat. No. 5,506,466. This technique has the problem that the beads and the irregular bent/curved portions of the shadow mask are projected onto the phosphor screen.
U.S. Pat. No. 4,136,300 discloses the idea of giving a curvature to the shadow mask by varying the pitch of the shadow mask with respect to a panel having flat inner and outer surfaces. However, this patent has no disclosure of a construction which includes an in-line type electron gun and a dot-type phosphor screen. In addition, this document describes a method of forming a curvature on a shadow mask in the case where the inner surface of the panel is flat and the shadow mask includes slots, but the document includes no description of practical problems and constructions.
A technique for changing electron beam spacing (so-called S-size) together with the deflection angle of an electron beam in order to allow the curvature of the shadow mask to be larger than that of the panel inner surface is described in the International Display Workship (IDW) 1998 at pp. 413-416. This technique requires an electromagnetic quadrupole for varying the S-size together with the deflection angle. U.S. Pat. No. 5,479,068 describes a technique for enlarging the vertical pitch of the apertures of a shadow mask to improve color purity tolerances and electron beam transmissivities in a Braun tube which has an in-line type of electron gun and a dot-type of phosphor screen. However, this document has no description of a shadow mask having a curved surface of the strength thereof.
The present invention relates to a color picture tube, such as that used in a TV broadcasting receiver set, of the type in which the outer surface of the panel is flat and no high resolution is needed, but in which the brightness of the screen is important. Specifically, the color picture tube according to the present invention includes a panel having an outer surface whose equivalent radius of curvature is 10,000 mm or more and a press-formed type of shadow mask, and the horizontal pitch of apertures in the center of the shadow mask is 0.6 mm or more. The structure of the phosphor screen is not a stripe-type, but is a delta type, and an electron gun is arranged in the color picture tube in an in-line manner. This construction makes it possible to easily vary the pitch of the shadow mask in the direction of the Y-axis, while enjoying the benefit of self-convergence of the in-line type electron gun, thereby realizing the required curvature of the shadow mask.
According to the present invention, in the above-described construction, in order that the required landing tolerance can be ensured and the required luminance of the phosphor screen can be ensured, the delta shape of the arrangement of the shadow mask apertures is made vertically long to prevent degradation of landing tolerances, other than the horizontal landing tolerance. In addition, the shape of each of the shadow mask apertures is made vertically long to ensure sufficient luminance, as required.